Computer displays and televisions are known to utilize different display techniques; computer displays utilize a progressive non-interlaced display technique, while televisions utilize an interlaced technique. The progressive display technique creates frames for displaying wherein each frame includes complete pixel information for the display. For example, if the computer display is a 640×480 pixel display, each line of a frame would include the appropriate pixel information for display. Thus, each frame display, which is done at a refresh rate of the computer display (e.g., 60 hertz, 75 hertz, 90 hertz, etc.), is complete. Because each frame contains complete pixel information, no human perception filtering is required.
The interlaced technique has two types of fields: top fields and bottom fields. Top fields and bottom fields are also known as odd and even fields. A top field contains only the pixel information for odd lines of the frame, while the bottom field contains only pixel information for even lines of the frame. As such, both fields are incomplete. When displayed at the refresh rate of the television, which may be approximately 60 hertz in North America, the fields are presented at a rate of 30 top fields and 30 bottom fields per second. Because the interlaced display technique alternates between top fields and bottom fields, the human visual process filters the fields such that complete images (i.e. frames) are perceived by the viewer.
Prior Art FIG. 1 illustrates the manner 100 in which the interlaced technique operates. Initially, pixel information for a plurality of source lines S0, S1, S2, S3, etc. of a source frame 101 is provided. From the source lines, a top field 102 of lines T0, T1, T2, T3, etc. is extracted along with a bottom field 104 of lines B0, B1, B2, B3, etc. for display on a destination frame 103.
As shown in Prior Art FIG. 1, T0 and T3 are copies of S0 and S2. Further, scaling is done to generate T1 and T2 from S0 and S2. Such scaling is accomplished by interpolation utilizing the equations shown. It should be noted that scaling is often used when it is desired to display a first number of source lines on a destination frame with a second number of lines greater than the first number. Similar to the top field 102, the scaling operation may also be performed on the bottom field 104.
With the source lines S0, S1, S2, S3, etc. scaled and the top field lines T0, T1, T2, T3, etc. and bottom field lines B0, B1, B2, B3, etc. generated, the top and bottom fields are adapted to be depicted in a destination frame 103 on an output device 106.
As set forth in FIG. 1, a raster scanout pattern of a destination frame 103 is shown to contain both top and bottom fields and a vertical blanking region. In particular, the top field lines T0, T1, T2, T3, etc. and bottom field lines B0, B1, B2, B3, etc. are projected on the destination frame 103 in accordance with sync waveforms 108. Such corresponding horizontal and vertical sync waveforms 108 are shown with a vertical blanking waveform 109, to show where the active lines are in the fields.
Table #1 illustrates various parameters used for the display configuration shown in FIG. 1.
TABLE #1TOTAL LINES IN DISPLAY FRAME: 13 (ODD NUMBER)TOTAL LINES IN DISPLAY FIELD: 6.5ACTIVE LINES IN DISPLAY FRAME: 8ACTIVE LINES IN DISPLAY FIELD: 4LINES IN BLANKING PERIOD: 3
Unfortunately, if flicker filtering is not employed, interlacing results in aliasing which is different in the two fields. This results in flickering.
Prior Art FIG. 1A illustrates an alternate technique 110 for generating the active lines of FIG. 1 from a source frame. In such alternate technique, scaling precedes interlacing, unlike the embodiment of FIG. 1. Unfortunately, a filter used to reconstruct the new lines prior to interlacing is typically a poor anti-aliasing filter. Subsequently, the interlacing results in aliasing, which is different in the two fields, resulting again in flicker.
During both techniques for generating active lines from a source frame (FIGS. 1 and 1A), the source frame can unfortunately only be changed once every two field scanout periods, if all of the information in the source frame is to be displayed since the source frame is used to generate both display fields. This restricts the frame animation rate to one half of the field display rate.
It should be noted that it is still possible to animate the source frames at a field scanout rate. To accomplish this, the top or bottom lines may be extracted as appropriate, as shown in the foregoing figures. The problem, however, is that only half the source frame information gets displayed.